1. Field of the Invention
The invention relates to a vehicle and a vehicle control method. More particularly, the invention relates to a vehicle provided with an internal combustion engine having a plurality of cylinders, and a control method of that vehicle.
2. Description of the Related Art
A vehicle that is driven by an internal combustion engine is typically provided with an exhaust gas purification catalyst and an air-fuel ratio sensor in an exhaust passage of the internal combustion engine. Moreover, to improve exhaust gas purification performance of the exhaust gas purification catalyst, the vehicle is also provided with a control apparatus that brings the air-fuel ratio of the internal combustion engine close or equal to a stoichiometric air-fuel ratio based on the detection results of the air-fuel ratio sensor.
Japanese Patent Application Publication No. 10-103139 (JP-A-10-103139), for example, describes one such control apparatus that includes air-fuel ratio sensors upstream and downstream of an exhaust gas purification catalyst. This control apparatus performs main feedback control of the air-fuel ratio according to the oxygen concentration in the exhaust gas detected by the upstream air-fuel ratio sensor, and executes air-fuel ratio control that corrects the control value of the main feedback control according to the oxygen concentration in the exhaust gas detected by the downstream air-fuel ratio sensor.
The related control apparatus described in JP-A-10-103139 performs main feedback control that controls the injection quantity of fuel supplied into the cylinders of the internal combustion engine according to the oxygen concentration in the exhaust gas detected by the upstream air-fuel ratio sensor, as well as performs so-called sub-feedback control that corrects the oxygen concentration in the exhaust gas detected by the upstream air-fuel ratio sensor according to the oxygen concentration in the exhaust gas detected by the downstream air-fuel ratio sensor.
Also, with the related control apparatus described in JP-A-10-103139, if a misfire, in which the mixture of air and fuel does not ignite, occurs in the internal combustion engine, the unburned air-fuel mixture is discharged into the exhaust passage. As a result, unburned HC reacts with the residual air in the exhaust gas purification catalyst, which may cause the exhaust gas purification catalyst to overheat. Therefore, the misfire rate of the internal combustion engine is calculated and the main feedback control value is corrected according to the calculated misfire rate.
In this kind of control apparatus, the pressure in the combustion chamber is detected by a pressure sensor and if the detected pressure is equal to or less than a predetermined value, it is determined that a complete misfire has occurred. Also, the misfire rate is obtained by sampling the number of times complete misfires have occurred per a predetermined number of times the cylinder has been fired, and dividing the sampled number of complete misfires by the predetermined number of times the cylinder has been fired.
However, with the related control apparatus of the internal combustion engine described in JP-A-10-103139, although the occurrence of complete misfires in the cylinders of the internal combustion engine is determined, the degree of combustion of the air-fuel mixture in the cylinders is not determined. Therefore, it is not possible to predict the degree of deterioration of the combustion state in the internal combustion engine. As a result, it is not possible to correct the control value of the air-fuel ratio control according to the degree of deterioration of the combustion state.